(1) Field of the Invention
The present invention relates to an induction synchronous motor of a permanent magnet type which starts by the induction motor action and is brought into a synchronous operation by permanent magnets.
(2) Description of the Related Art
A conventional motor of the kind to which the present invention relates is a synchronous motor having a brushless configuration. In such a conventional motor, a unitary rotor comprises a squirrel cage type conductive portion of the induction motor and a rotor portion of permanent magnets. The motor starts by the magnetic induction action produced between the rotating magnetic field of the stator and the squirrel cage type conductive portion of the induction motor and, when the rotating speed accelerates from the asynchronous speed of the induction motor and approaches the synchronous speed, that is, when the slip approaches zero, the rotating magnetic field of the stator and the magnetic poles of the permanent magnets attract each other thereby changing the motor speed to its synchronous speed.
In the conventional synchronous motor explained above, the motor starts by the magnetic induction action between the rotating magnetic field of the stator and the squirrel cage type conductor portion of the induction motor. During the asynchronous operation under this state, there are produced at the same time an attracting action and a repelling action between the permanent magnets and the rotating magnetic field produced by the stator. This necessitates the squirrel cage type conductive portion to have a magnetic induction action fully sufficient to overcome the starting interference caused by the simultaneous attraction and repulsion between the permanent magnets and the rotating magnetic field of the stator during the asynchronous speed operation. Thus, the squirrel cage type conductive portion of the induction motor is required to be sufficiently large to overcome and to make negligible the starting interference of the permanent magnet rotor. On the other hand, the synchronous torque which is inherently required depends on the size of the permanent magnets. In order to enable the starting torque of the induction motor to be sufficiently large to overcome the action of the permanent magnets, it has been required that the permanent magnets be kept small. For this reason, in comparison of the size of the motor with its output, a brushless synchronous motor of a permanent magnet type has remained large in size.
The brushless induction motor of a permanent magnet type as explained above is often used in recording or reproduction devices of small sizes in which WOW is particularly undesired. The problems as explained above place limits to making such devices more compact so that there is a demand and desire for the realization of a synchronous motor which is small in size but its output remains large.